The present invention relates to a measuring device for measuring a measurement object, comprising an illumination device for illuminating the measurement object with an illumination pattern, a pattern generation device with at least one pattern generating element for bringing about a positionally variant intensity distribution of the illumination pattern, and an optical sensor arrangement for detecting the illumination pattern reflected and/or scattered by the measurement object.
Furthermore, the present invention relates to an apparatus for measuring a measurement object comprising such a measuring device.
Furthermore, the present invention relates to a method for measuring a measurement object.
The present invention is generally concerned with contactless optical sensors for topologically detecting surfaces of measurement objects. The main areas of use for such sensors are industrial metrology, i.e. the metrological measurement of mechanical workpieces. In principle, however, sensors of this type can be used in all imaging operations.
One advantage of contactlessly measuring optical sensors of this type resides primarily in the fact that a surface can be measured significantly more rapidly in comparison with a sensor that effects tactile measurement.
Such measuring systems that effect tactile measurement are known in principle in the prior art. In this case, the surface topology to be measured is touched with the aid of a probe head or measuring probe. The measuring head has a touch element, e.g. a ruby sphere, and is part of a coordinate-measuring machine that brings the probe head with the touch element fixed therein in contact with the surface to be measured. The position and pressure force of the probe head or of the probe element is determined by means of strain gauges or other sensors. The surface contour is determined by punctiform or continuous tactile sensing. However, the speed of such systems that effect tactile measurement is limited. This arises, firstly, by virtue of the fact that a mechanical contact with the surface to be measured is always required and, secondly, the measurement effected is only ever a punctiform measurement. Although provision can indeed be made for automatically traversing a path on the surface in the context of a so-called “scanning process”, even this is only a sequence of punctiform measurements. The size of the probe element additionally limits the measurable radii of the surface, i.e. small depressions generally cannot be detected. Furthermore, under certain circumstances, very hard probe elements can cause damage to very sensitive surfaces.
Furthermore, monochromatic confocal sensors have been proposed which focus monochromatic light, generally a laser beam, into the vicinity of the surface to be measured. The light reflected or scattered back from the surface to be measured is imaged onto a pinhole diaphragm in or in the vicinity of a confocal plane. The light power transmitted by the pinhole diaphragm is a measure of the distance between the object-side focal plane and the surface. By way of example, by keeping constant the transmitted light power, the surface can always be kept at the focus of the sensor. The surface topology can then be detected by point-by-point measurements along a predefined path in a manner similar to that in the case of a tactile sensor. Accordingly, the measurement here is also effected only in punctiform fashion and the requirements made of the control precision and the operating distance to be set are relatively precise.
Chromatic confocal sensors are also known in which a spectrally wide-band light is focused into the vicinity of the surface to be measured. The focusing is effected by means of an optics having a large longitudinal chromatic aberration, i.e. the focal plane varies greatly depending on the wavelength of the light. Likewise with the aid of a confocal pinhole diaphragm and a spectrometer, the distance between the sensor and the surface can be determined over a relatively large operating distance range, the length of which results from the difference between the object-side vertex focal length of the largest and smallest wavelengths of the wide-band light. Here, too, the measurement is, however, effected in each case only in punctiform fashion, as is shown for example in the document EP 1 287 311 B1.
Furthermore, so-called stripe illumination methods or deflectometry methods are known which usually project stripe patterns onto the surface to be measured. The surface topology to be measured deforms the stripes reflected by the surface. The reflected stripes are detected by means of a camera, i.e. a two-dimensional sensor array. The surface topology can be deduced from the measured deformation of the reflected stripe pattern. A simple variant of stripe illumination is laser line illumination, in which the form of a laser line radiated onto the measurement object is then evaluated in each case. Although a larger area can be detected and measured by this method, in return the measurement accuracy is lower than in the case of the punctiform measuring methods mentioned above. Furthermore, in the case of highly scattering surfaces, problems can occur on account of ambiguities in the evaluation.
Furthermore, interferometric methods are known in which the surface to be measured is irradiated with coherent or partly coherent light and is viewed in reflection. The surface topography then influences the light interference in terms of the depth, i.e. in the direction of propagation, or laterally, i.e. perpendicular to the direction of propagation. Interferometric methods with narrowband laser light are suitable for example for the high-precision measurement of lens surfaces. Spectrally wide-band light is used in the case of so-called white light interferometry or optical coherence tomography (OCT). The class of interferometric methods overall also includes holography. Consequently, all interferometric methods also require a reference beam path, which, however, always has a high sensitivity to disturbance in the measurement set-up.
Some of the methods mentioned above effect only punctiform measurement. The methods that effect areal measurement always operate with an angle between the illumination direction and viewing direction. However, in particular spatially greatly restricted applications which allow only limited viewing angles with respect to the surface to be measured, for example the measurement of bores or channels, do not allow large angles between the illumination direction and viewing direction, and so the precision of the methods that effect areal measurement thus also decreases.
Therefore, the document DE 199 55 702 A1 proposes an optical measuring method in which a luminous point or a ring is radiated as illumination pattern in the direction of the surface to be measured. Depending on whether the surface to be measured lies in the pattern, specific reflection figures arise, which can then be evaluated. From a possible distortion of the pattern, it is furthermore possible to determine not only the distance between the measuring point and the sensor but also an inclination of the probed surface point and thereby to deduce points lying around the measured point and their position.
Furthermore, the document DE 34 18 767 A1, for example, proposed using a specific aberration, the astigmatism, for the punctiform measurement of surface topographies.
Although the methods disclosed in the two documents mentioned above outline measuring systems in which the illumination direction and the viewing direction can run substantially parallel and coaxially with respect to one another, they yield a precise measurement only for a point respectively lying in the center of the respective sensor recording.
Therefore, it is an object of the present invention to provide a measuring device which eliminates the disadvantages of the methods mentioned above and enables a higher measurement speed in particular by means of a measurement that is not only punctiform.